A conventional internal combustion engine is configured such that cylinders have uniform displacements in order to satisfy the distribution characteristics required by intake and exhaust systems. The uniform displacement engine has an advantage in that it is possible to easily control the air-to-fuel ratio and exhaust gas. However, the uniform displacement engine has a disadvantage in that the margin for operation point control is insufficient due to the fixed displacement thereof. As a result, it is difficult to satisfy both operational efficiency and fuel efficiency within a specific operation zone, and therefore problems must be addressed by compromising one or both of operational efficiency and fuel efficiency.
In addition, in the engine having fixed displacement cylinders, excessive mechanical energy is consumed in order to secure stable idling, which is inefficient. In particular, operation point control is inevitably and frequently inefficient due to the limitations imposed by vibration and noise.
Such problems, which frequently occur over the entire operation zone of the conventional internal combustion engine, may be solved by striking a tradeoff between operational efficiency, fuel efficiency and emissions.
Meanwhile, Japanese Patent Laid-Open Publication No. 2007-162672 discloses a cylinder arrangement structure of a multi-stage type displacement adjustable engine in which cylinders having different displacements are arranged such that the displacement of the engine is adjusted in multiple stages based on operational combinations of the cylinders.
The cylinder arrangement structure of the multi-stage type displacement adjustable engine disclosed in Japanese Patent Laid-Open Publication No. 2007-162672 has effects in that the displacement of the engine is adjusted in multiple stages, and therefore it is possible to control the operation of the engine in respective operation zones, thereby simultaneously improving operational efficiency and fuel efficiency, but has problems in that vibration and noise are generated due to imbalance in displacement among the cylinders, making it difficult to apply the cylinder arrangement structure of the multi-stage type displacement adjustable engine to vehicles that are actually mass-produced.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.